Both cypermethrin (U.S. Pat. No. 4,024,163) and cyfluthrin (U.S. Pat. No. 4,218,469) are well-known pyrethroid insecticides and are articles of commerce in many countries. Each may contain eight isomers.
For racemic cypermethrin the eight isomers are designated I-VIII as follows:
______________________________________ cis isomers I = ( .sub.-- S)-(cyano)(3-phenoxyphenyl)methyl -1 .sub.-- R,cis-3-(2, 2-di- chloroethenyl)-2,2-dimethylcyclopropanecarboxylate (abbreviated -1 .sub.-- R,cis S) II = ( .sub.-- R)-(cyano)(3-phenoxyphenyl)methyl --1S,cis-3-(2,2-di- chloroethenyl)-2,2-dimethylcyclopropanecarboxylate (abbreviated --1S,cis .sub.-- R) III = ( .sub.-- S)-(cyano)(3-phenoxyphenyl)methyl --1S,cis-3-(2,2-di- chloroethenyl)-2,2-dimethylcyclopropanecarboxylate (abbreviated --1S,cis .sub.-- S) IV = ( .sub.-- R)-(cyano)(3-phenoxyphenyl)methyl -1 .sub.-- R,cis-3-(2, 2-di- chloroethenyl)-2,2-dimethylcyclopropanecarboxylate (abbreviated 1R,cis .sub.-- R) trans isomers V = ( -1 .sub.-- R,trans .sub.-- S), the trans isomer of I VI = ( --1S,trans .sub.-- R), the trans isomer of II VII = ( --1S,trans .sub.-- S), the trans isomer of III VIII = ( -1 .sub.-- R,trans .sub.-- R), the trans isomer of ______________________________________ IV
The designations 1R,cis; 1R,trans; 1S,cis: and 1S,trans refer to the spatial relationship of the hydrogen atoms at the 1 and 3 positions of the cyclopropane ring, and the single letter designations R and S refer to the spatial configuration of the cyano group on the alpha carbon of the alcohol portion of the molecule.
The same designations are applied to the corresponding isomers of cyfluthrin.
It is well known that the most insecticidally active isomers are I and V. It is also well known that the enantiomer pairs I/II, referred to as the c.sub.2 enantiomer pair, and V/VI, referred to as the t.sub.2 enantiomer pair, are more insecticidally active than the enantiomer pairs, III/IV, referred to as the c.sub.1 enantiomer pair, and VII/VIII, referred to as the t.sub.1 enantiomer pair.
It is extremely difficult and commercially impractical to separate the more active isomers from the complex mixture of isomers produced in the usual synthesis of pyrethroids. Consequently, substantial effort has been devoted to methods for increasing the proportion of active isomers in products consisting of mixtures of isomers, e.g., by converting less active isomers in such mixtures to more active isomers. Such processes are disclosed in U.S. Pat. Nos. 4,427,598 and 4,782,174, inter alia.
While the increased processing costs incurred in such isomer enrichment techniques may be justified for some pesticidal applications, an esterification process which would lead directly to a mixture enriched in the more active isomers would be highly desirable, both for the increased insecticidal activity of the esterification product itself and as an enhanced feed stock for conversion.